U.S. patent number 6,470,193 [Application Number 08/843,006] was granted by the patent office on 2002-10-22 for power efficient indoor radio base station.
This patent grant is currently assigned to Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Tomas Nils Stolt.
United States Patent |
6,470,193 |
Stolt |
October 22, 2002 |
Power efficient indoor radio base station
Abstract
A power efficient indoor radio base station for use with
wireless cellular telecommunication systems is disclosed. A small
unitary package design is achieved by reducing the level of power
dissipation, and subsequently, the size of the heat sink required
for heat dissipation. In a preferred embodiment of the present
invention, a plurality of transmitter signals are combined in a
hybrid combiner to generate a pair of output signals. A first
output signal is transmitted through a dipole antenna resulting in
vertical polarization and a second output signal is transmitted
through a horizontal antenna producing horizontal polarization.
Further, a phase shift of 90.degree. is introduced between the
signals prior to transmission. The resulting simultaneous
transmission of the perpendicularly oriented signals yields a
substantially circular polarized field in the area of coverage.
Power dissipation is reduced by transmitting the horizontal
polarized signal because it is converted to useful energy that
would otherwise be dissipated in a load resistor.
Inventors: |
Stolt; Tomas Nils (Stockholm,
SE) |
Assignee: |
Telefonaktiebolaget L M Ericsson
(publ) (Stockholm, SE)
|
Family
ID: |
25288811 |
Appl.
No.: |
08/843,006 |
Filed: |
April 11, 1997 |
Current U.S.
Class: |
455/562.1;
375/299; 455/103; 455/101 |
Current CPC
Class: |
H01P
1/2131 (20130101); H04B 1/38 (20130101); Y02D
70/144 (20180101); H04W 88/08 (20130101); Y02D
30/70 (20200801); Y02D 70/40 (20180101) |
Current International
Class: |
H04B
1/38 (20060101); H01P 1/213 (20060101); H01P
1/20 (20060101); H04Q 7/30 (20060101); H04B
001/38 (); H04B 001/02 (); H04L 027/04 () |
Field of
Search: |
;455/522,574,101,103,105,560,561,562 ;375/299 ;343/797 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
713259 |
|
May 1996 |
|
EP |
|
0 713 259 |
|
May 1996 |
|
EP |
|
2304496 |
|
Mar 1997 |
|
GB |
|
WO95/34102 |
|
Dec 1995 |
|
WO |
|
Other References
European Standard Search Report re RS 99342. Date of completion of
search: Sep. 16, 1997..
|
Primary Examiner: Vo; Nguyen T.
Assistant Examiner: Appiah; Charles N.
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Claims
What is claimed is:
1. A radio base station for use with a wireless telecommunication
system comprising: a first transmitter for transmitting a first
transmitter signal; a second transmitter for transmitting a second
transmitter signal; a combiner including at least two input ports
coupled to the first transmitter and the second transmitter for
combining said first and second transmitter signals, wherein said
combiner generates a first output combiner signal and a second
output combiner signal having a phase shift of 90.degree. with
respect to each other; and a first antenna and a second antenna
oriented perpendicularly to each other and coupled to the combiner
and configured for simultaneously transmitting said first and
second output combiner signals through the first antenna and the
second antenna respectively such that the emitted signals form a
substantially circular polarized field such that said first and
second output combiner signals are transmitted with reduced waste
energy.
2. A radio base station as recited in claim 1 wherein said combiner
is a hybrid combiner.
3. A radio base station as recited in claim 2 wherein a duplex
filter is coupled between the hybrid combiner and the at least one
antenna.
4. A radio base station as recited in claim 3 wherein said first
antenna and second antenna are oriented as a vertical antenna and a
horizontal antenna.
5. A radio base station as recited in claim 3 wherein said first
antenna and said second antenna is a patch antenna.
6. A radio base station as recited in claim 5 wherein said first
and second output combiner signals are emitted from the patch
antenna having vertical and horizontal polarized orientations
respectively.
7. A radio base station as recited in claim 1 wherein the
substantially circular polarized field is elliptically
polarized.
8. A radio base station as recited in claim 1 is housed in a
weatherproof cabinet for outdoor use.
9. A radio base station as recited in claim 1 wherein additional
transmitters are included for increased channel capacity.
10. A method of transmitting signals from a radio base station for
use in a cellular telecommunication network, comprising the steps
of: combining a plurality of transmitter output signals with a
combiner said combiner including at least two input ports for
receiving said plurality of transmitter output signals; generating
first and second combiner output signals using said signals
received on said at least two input ports with the combiner,
wherein a relative phase shift of 90.degree. is introduced between
the signals; and transmitting the first and second combiner output
signals simultaneously through a first antenna and a second antenna
perpendicularly oriented to each other such that one signal is
transmitted with vertical polarization and the other signal with
horizontal polarization to produce a substantially circular
polarized field, wherein the transmission of the first and second
combiner signals represents a substantial majority of signal energy
from said plurality of transmitter output signals that is
ultimately transmitted thereby reducing the amount of power
dissipation in the base station.
11. A method as recited in claim 10 wherein said combining is
performed with a hybrid combiner.
12. A method as recited in claim 10 wherein said first and second
combiner output signals are transmitted through a vertical antenna
and a horizontal antenna.
13. A method as recited in claim 10 wherein said first and second
antennas are comprised of a patch antenna.
14. A method as recited in claim 13 wherein the radio base station
transmits as said substantially circular polarized field an
elliptically polarized field by varying at least one of the
magnitude and phase of the first and second combiner output
signals.
15. A method as recited in claim 10 wherein said power dissipation
is removed by a heat sink.
Description
FIELD OF INVENTION
The present invention relates generally to radio base stations used
in wireless telecommunication systems. In particular, it pertains
to a small low-heat dissipating radio base station that is
especially suitable for indoor applications.
BACKGROUND OF THE INVENTION
The explosive growth in the wireless telecommunications industry
has fueled the demand for a vast array of telecommunication
services that are either currently being offered or planned for
implementation. These services include traditional analog and
digital cellular, and Personal Communication Services (PCS) that
include voice, paging, data, and fax capabilities. By many
indications, these services will become increasingly popular in the
coming years leading, in all likelihood, to expectations of higher
levels of service. For example, the ability to access these
services from more and more locations becomes an increasingly
important issue. Furthermore, the search for more revenue has
service providers increasingly interested in being able to provide
access to their services in areas that were previously
inaccessible. For example, it would be desirable to provide
coverage in previously untapped regions such as large indoor areas
due to the lack of coverage from conventional outdoor equipment.
Such regions may include hotel lobbies, subway stations,
restaurants, convention and entertainment centers, office buildings
and other situations where localized wireless coverage is required
or where subscriber concentrations and call volumes are high.
In a cellular telecommunication system, a mobile switching center
(MSC) is linked to a plurality of base stations that are
geographically dispersed to form the area of coverage for the
system. The radio base stations (RBS) are designated to cover
specified areas, known as cells, in which two way radio
communication can then take place between the mobile station (MS)
and RBS in the coverage area. Although originally conceived for
outdoor environments, this idea can be adapted to provide indoor
coverage by installing radio base stations in these indoor areas.
These RBSs are typically smaller than the outdoor variety and
provide coverage by creating micro cells over the region.
Although performance of these indoor systems have been adequate,
there are some drawbacks with the design of existing RBSs. For
example, it is desirable to reduce the size of the indoor base
stations further so that they would be much more unobtrusive and
simpler to mount. Very small RBSs, in addition to enhancing
aesthetics, allows for simplified mounting and reduces installation
costs. For example, very small RBSs would be able to be mounted on
existing structures, support beams, or mounted on a wall as opposed
to requiring dedicated support structures or special mounting
arrangements. One major factor that has inhibited reduction of RBSs
to very small sizes has been the relatively large heat dissipating
devices required for proper operation.
FIG. 1 shows a perspective view of a prior art Ericsson RBS 884
Micro Radio Base Station 10. Micro Base Station 10 was designed to
provide localized coverage in the form of micro cells for indoor
environments and is essentially a scaled-down version of base
stations used outdoors. The interior components of Base Station 10
are housed in metal cabinet 12 measuring approximately 440
mm.times.310 mm.times.488 mm (17.2 in.times.12.2 in.times.19.2 in),
the separately installed antennas are not shown. A disadvantage of
this base station is that its size makes unobtrusive installation
difficult and inconvenient. Further, the antenna structure must be
mounted separately making installation more complex and expensive.
Furthermore, the heat sink required for proper operation of the
internal circuit components, which may include built-in fans, is
the limiting factor in reducing the size of the base station. The
operation and heat removal requirements of the internal circuit
components of base station 10 are described herein.
FIG. 2 shows a functional block diagram of the Micro Base Station
10 of FIG. 1. The output of transmitter TX114 is combined with
transmitter TX216 with a hybrid combiner 18. The output of combiner
18 yields two components: a component 19 which is subsequently used
for transmission and component 21 which is not transmitted but
terminated in load resistor 24. Load resistor 24, shown separately
from combiner 18 for simplicity, provides matching impedance for
combiner 18 to minimize reflections for increased transmission
efficiency. After emerging from combiner 18, component 19 is sent
to a duplex filter 20 and then is routed to a dipole antenna 22 for
transmission through the air. The component 21, after emerging from
combiner 18, is dissipated as heat in load resistor 24. Roughly
half of the total power emerging from combiner 18 is sent on for
transmission (component 19) and the other half is dissipated in
load 24 (component 21). Therefore, a signal loss of approximately a
little more than 3 dB is typically experienced due to combiner 18
and load resistor 24. Similarly, transmitter TX326 and transmitter
TX428 are combined in hybrid combiner 30 where the transmitted
component is sent to duplex filter 32 and then to antenna 34.
Similarly, the non-transmitted component from combiner 30 is
terminated in load 36 and dissipated as heat. By way of example,
the combination of 400 mW signal from TX1 and 400 mW from TX2 into
combiner 18 results in approximately 100 mW per carrier of power
transmitted from antenna 22 and 400 mW dissipated in load 24 as
heat. With a comparable figure of 400 mW requiring dissipation in
load 36 from TX3 and TX4, it becomes apparent that a relatively
sizable heat sinking capacity capable of dissipating at least 800
mW is required for proper operation.
In view of the foregoing, it is an objective of the present
invention to provide a technique for reducing the amount of heat
dissipation required while maintaining substantially the same
coverage area as compared to a base station with a terminated load.
Further, as will be described hereinafter, the present invention
provides a method and apparatus for constructing an indoor
multi-carrier radio base station that is small, unobtrusive, and
simple to install.
SUMMARY OF THE INVENTION
Briefly described, and in accordance with multiple embodiments
thereof, the invention provides a technique for reducing heat
dissipation in indoor radio base stations. In a first embodiment of
the invention, a low-heat dissipating radio base station is
provided comprising first and second transmitters with their output
signals coupled to and combined with a hybrid combiner. The
combiner generates a first output combiner signal to be transmitted
through a dipole antenna, which produces vertical polarization, and
a second output combiner signal transmitted through a horizontal
antenna producing horizontal polarization. Prior to transmission,
the output combiner signals are shifted in phase by 90.degree. with
respect to each other by the combiner. The resulting transmission
of the perpendicular oriented signals produces a substantially
circular polarized field in the area of coverage. Alternatively, an
elliptically polarized field may be produced by varying the
magnitude and/or phase of the emitted signals.
In a method aspect of the present invention, a method of reducing
the power dissipated, and subsequently the size, of a radio base
station is disclosed. The method includes combining a pair of
transmitter output signals with a hybrid combiner. The combiner
generates a first combiner output signal and a second combiner
output signal. A phase shift of 90.degree. is introduced by the
combiner between the output signals. The combiner output signals
are arranged to be emitted from an antenna such that the
orientation of the signals are perpendicularly oriented to form a
substantially circular polarized field. The transmission of the
circular polarized field eliminates the need for signal termination
in a heat dissipating load thereby reducing heat dissipation in the
base station.
The embodiments of the present invention provide an efficient
low-power consuming unitary radio base station in a small,
convenient package. The small package design facilitates simpler
mounting for unobtrusive, aesthetically pleasing installation.
Further, the circular polarized field provides improved reception
at the receiving station in the field of coverage. These and other
advantages of the present invention will become apparent upon
reading the following detailed descriptions and studying the
various figures of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages
thereof, may best be understood by reference to the following
description taken in conjunction with the accompanying drawings in
which:
FIG. 1 is a perspective view of a prior art indoor base
station;
FIG. 2 is a functional block diagram of the indoor base station in
FIG. 1;
FIG. 3 is a functional block diagram of a base station in
accordance with a first embodiment of the present invention;
FIG. 4 is a functional block diagram of a base station in
accordance with a second embodiment of the present invention;
and
FIG. 5 is a perspective view of an indoor base station in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A discussion of FIGS. 1 and 2 directed toward a prior art indoor
radio base station was provided in the preceding sections.
Referring now to FIG. 3, a simplified functional block diagram of a
multi-carrier radio base station (RBS) is shown in accordance with
a first embodiment of the present invention. For sake of
simplicity, the receiving portions of the transceiver assemblies in
the RBS have been omitted.
In cellular telecommunication systems, the transmitters operate at
designated radio frequencies and are separated by a frequency
distance as determined by a specified standard such as Advanced
Mobile Phone Standard (AMPS). The power level of each of the output
signals from transmitters TX140 and TX242 effectively determines
the distance the signal is radiated or size of the coverage area.
To make efficient use of a common antenna, signals from multiple
transmitters are collected and routed to the antenna prior to
transmission. A typical method used for collecting the transmitter
output signals is by utilizing a combiner. In accordance with the
first embodiment, a hybrid combiner 44, comprising a mixture of
waveguides and passive electronics to achieve combination of the
signals, is used. As those skilled in the art can appreciate,
hybrid combiners are much smaller than the large cumbersome
mechanical resonators used in the past, such as filter or cavity
combiners. Further, hybrid combiners operate over a much broader
range of frequencies and therefore do not require tuning. The
drawback is that hybrid combiners are less efficient than
mechanical resonators and typically cause losses of approximately a
little more than 3 dB. This energy loss is radiated as heat
imposing further demands on the heat dissipating system.
The combination of signals from TX140 and TX242 in the hybrid
combiner 44 yields two components. Combiner 44 introduces a phase
shift of 90.degree. between the two components, in a procedure that
is well known in the art. A first signal component 45 emerges from
combiner 44 having a relative phase value of zero degrees and is
subsequently sent to the duplex filter 46. Upon leaving the duplex
filter, the signal is sent to dipole (vertical) antenna 48 for
transmission, whereby the emitted signal has a vertically polarized
orientation. Duplex filter 46 serves the purpose of separating the
transmitted signals from the received signals so that a common
antenna can be used. The relatively small duplex filters utilized
in the illustrated embodiment have losses of approximately 3 dB. A
second signal component 47 leaves combiner 44, having a phase shift
of 90.degree. with respect to the first signal component 45, and is
sent to horizontal antenna 50 for transmission. The resulting
emitted signal from antenna 50 has a horizontally polarized
orientation. The input impedance of antenna 50 is chosen to be
equivalent to the matching impedance of the combiner 44 to minimize
signal reflections. In the prior art, the second component is
unsuitable for transmission through a second vertical antenna
because of interference between antennas of the same polarization
that are in close proximity. This coupling between antennas causes
undesirable signal distortion and degradation of field (destructive
interference) which is well known to those in the art.
The simultaneous transmission of perpendicularly oriented signals,
such as vertically and horizontally polarized signals, are
theoretically uncoupled and thus do not interfere with each other.
In practice, it has been found that some coupling exists but there
is at least a 25-30 dB insulation between the vertical and
horizontal antennas. The simultaneous transmission of vertical and
horizontal oriented signals, of equal magnitudes with a phase
difference of 90.degree., is known to those in the art as circular
polarization. It should be apparent to those skilled in the art
that circular polarization is a special case where the conditions
of perpendicular orientation, equal magnitude, and phase 90.degree.
shift are met, and wherein variations of magnitude and/or phase
will yield an elliptically polarized field. A major advantage of
transmitting a circular polarized field from a radio base station
(RBS) is that the second component output from the combiner, which
is normally dissipated, is instead transmitted as useful energy.
Therefore, increases in the overall efficiency of the RBS are
realized since less energy is wasted. With increased efficiency,
less output power is required of transmitters 40 and 42 to achieve
the same or substantially similar coverage as in the prior art.
This leads to even a further reduction in heat removal requirements
for the system. Consequently, a much smaller heat sink is required
thereby permitting the RBS unit to be much smaller.
Another advantage of emitting a circular polarized field is that
mobile station (MS) reception is more robust. By way of example,
the field produced by a dipole antenna is received by the MS
reliably when the MS is positioned vertically i.e. the vertical
antenna in the MS matches the orientation of the field. When the MS
is moved out of the vertical plane, the signal starts to fade and
reception becomes weaker. This is caused by the antenna of the MS
moving into a null in the broadcast field. In contrast, an MS in a
circular polarized field is capable of receiving the signal equally
well in the vertical and horizontal planes and all planes in
between. This increases the probability of good reception while
using the MS in various positions such as while lying down, for
example. In a field containing elliptical polarization, the
strength of reception is not uniform but is a function of the
position of the MS. Therefore, the angle at which the best
reception is achieved may be skewed.
FIG. 4 shows a simplified functional block diagram of a second
embodiment in accordance with the present invention. Increased
carrier (channel) capacity is achieved by the addition of
transmitters TX354 and TX456 to the RBS of FIG. 3. Any number of
transmitters may be added to the system to increase channel
capacity. Transmitters TX1, TX2, TX3 and TX4 operate at distinct
frequencies thereby providing the RBS with a four-carrier
transmitting capability. In similar fashion to FIG. 3, the outputs
of TX354 and TX456 are combined in hybrid combiner 58. Similarly,
the outputs of combiner 58 are comprised of components 59 and 60
which are phase shifted 90.degree. relative to each other.
Component 59 is routed through duplex filter 62 and then to dipole
antenna 64 to be emitted with a vertically polarized orientation.
The component 60 is sent to horizontal antenna 66 for transmission
having horizontal polarization. Both of the radiated components
combine in the air to form a substantially circular polarized field
in the area of coverage. It should be noted that a theoretically
perfect circular polarized field is not attainable in practice,
therefore some variation of the signal that is substantially
circular is transmitted as a result. Circular polarization provides
a form of transmitting diversity that improves reception at the MS
handset for the aforementioned reasons.
In the embodiments described above, separate antennas were shown
for transmitting the vertical and horizontal polarized signals
respectively. In practice, a preferred embodiment may include a
patch antenna that can be used in place of, or in conjunction with,
a dipole antenna. Patch antennas, generally used in low powered
devices, are known to those skilled in the art as having desirable
properties and field emitting characteristics. By way of example,
patch antennas can be formed from a flat rectangular piece of metal
which can be relatively tiny. Tiny antennas facilitate their
incorporation into small base stations to form a functional unit
with no separate antennas to mount. When used with a backing plate,
patch antennas exhibit directional field emitting characteristics.
The field radiates from one side of the antenna only i.e. the base
station can emit a field directly in front of it but not behind it.
This situation may be very desirable in the operation of indoor
base stations. For example, a small indoor base station emitting a
circular polarized field with a patch antenna may be mounted
against the inside wall of a building. This permits coverage
throughout a room but not outside the building or into an adjacent
room behind it. In contrast, a base station with a dipole antenna
emits a radial field such that it must be positioned in the center
of the room for similar coverage. This may be undesirable since
mounting a base station in the center of a room may be more
conspicuous and/or less convenient.
As described above, power dissipation of the RBS is reduced in the
present invention by transmitting both output signals emerging from
the combiner whereas, in the prior art, one signal is terminated
and radiated away as heat. In turn, the amount of heat dissipation
required necessitates a relatively sizable heat sink that affects
the size of the base station. By way of example, in the prior art
(FIG. 2), to achieve 100 mW of radiated power at the antenna, 400
mW must be supplied to each transmitter TX1 and TX2. Accordingly,
400 mW is fed in matching load 24 to be dissipated. A similar
transmission output of 100 mW can be attained using the disclosed
inventive concept by radiating, for example, a 33 mW vertically
polarized and 67 mW horizontally polarized signal, thereby
requiring approximately 133 mW from each transmitter. It follows
that power dissipation is substantially reduced and the size of the
heat sink required may be dramatically reduced. It should be noted
that the unequal magnitudes of the signals are a result of losses
incurred in the duplex filter which will therefore produce an
elliptically polarized field. A circular polarized field may be
achieved by introducing a band-pass filter, for example, in the
horizontal antenna path to equalize the magnitudes but will result
in less than 100 mW total output power. To achieve a 100 mW
circular polarized field, the output power of the transmitters is
increased but will remain well below that of the prior art.
FIG. 5 is a perspective view of a low heat-dissipating,
reduced-size indoor radio base station constructed in accordance
with the present invention. The relatively small fins 70 are
conveniently hidden behind housing 72 for an inconspicuous,
aesthetically pleasant appearance. Further, the unitary
construction provides access to an array of simplified installation
options. For example, the unit may be positioned on a wall, support
column, girding, or even ceiling for concealed, unobtrusive
mounting that is both economical and efficient.
Although the invention has been described in some respects with
reference to specified preferred embodiments thereof, variations
and modifications will become apparent to those skilled in the art.
In particular, additional transmitters may be added in accordance
with the inventive concept to obtain supplemental calling capacity.
Further, the disclosure is not limited to indoor use since the
embodiments can easily be adapted to a weatherproof cabinet for use
outdoors. It is therefore, the intention that the following claims
not be given a restrictive interpretation but should be viewed to
encompass variations and modifications that are derived from the
inventive subject matter disclosed.
* * * * *